In general, electrically insulating ceramic materials are made of alumina as a major component as described in U.S. Pat. Nos. 3,423,517 and 3,723,176. However, alumina has the following defects: (1) it has a relatively large permittivity which causes a delay in signal propagation; (2) it has a high coefficient of thermal expansion which differs greatly from that of silicon semiconductor chips; (3) it has a high firing temperature which makes it difficult to achieve simultaneous firing with good electrical conductors such as gold, silver and copper. Alumina substitutes that meet the recent demand for the high-speed production of electrically insulating ceramic materials with high density and have low permittivity, low coefficient of thermal expansion comparable with that of silicon semiconductor chips, and firing temperature are necessary. Glass, for example, glass as described in Japanese Patent Publication No. 6257/82 and glass-ceramic product as described in Japanese Patent Application (OPI) No. 111517/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") as one of these substitutes has been studied by many researchers. However, glass is weak with respect to mechanical shock, cannot be easily formed to exact dimensions and suffers a great power factor tan .delta.. These inherent defects make glass unsuitable for use as electrical insulators.